This invention relates to apparatus and a method for remotely sensing the differential tire pressure of vehicles as they pass a checkpoint. Abnormal tire pressure in large vehicles such as trucks and busses constitutes a serious safety hazard in that low tire pressure can cause such things as sudden tread separation, separation of tires and wheels, or in the case of many trucks, explosive separation of wheel safety rims. High tire pressure can cause blow-outs. In the case of aircraft, tire failure can result in loss of control during landing or takeoff, with the serious risk of personal injury and/or property damage.
Prior art methods of tire pressure measurement include the use of a pressure gauge manually applied to each tire valve, or, the custom among some truckers of kicking of tires to find the ones which "sound" low in pressure. These methods are slow and unreliable and require that the vehicle be stopped to check the pressure. Many commercial vehicles travel long distances between stops and new regulations permit the carrying of heavier loads, which makes the consequences of unsafe tire pressures more critical.
The prior art also includes apparatus for automatically measuring the tire pressures of moving vehicles as the vehicle passes an instrumented checkpoint. Such apparatus usually requires that the vehicles carry a tire pressure sensor for each tire, and as the vehicle passes the checkpoint, the sensors are all interrogated by means of a radio transceiver which has its antenna mounted in the roadway at the checkpoint. An example of such apparatus is found in U.S. Pat. No. 4,067,235, issued to Markland on Jan. 10. 1978. Another similar system is shown in the Venema U.S. Pat. No. 4,363,020, issued on Dec. 17, 1982. In the Venema patent, an interrogating signal is transmitted from the vehicle to passive pressure transponders mounted on each wheel. The signals received from the transponders are an indication of tire pressure and abnormal pressures can be displayed to the vehicle's driver.
U.S. Pat. No. 4,355,299 issued to Cook on Oct. 19, 1982 shows a system for detecting low pressure tires in dual-wheeled vehicles in which the dual wheels pass over a plurality of liquid-filled flexible hoses which are disposed parallel to the direction of vehicle travel at a checkpoint. Low pressure tires are detected by comparing the pressure patterns in the hoses with reference patterns for normal inflation. The disadvantages of this system are that a large number of normal pressure patterns for a large number of makes and models of vehicles to be checked must be stored in the apparatus, and the make and model of the vehicle being checked must be determined before the test is made. Also, the pressure in the hoses is a function of the weight and loading of the vehicle and this can cause inaccurate results.